Selenium containing electrodeposition solution and methods

ABSTRACT

The present inventions relate to selenium containing electrodeposition solutions used to manufacture solar cell absorber layers. In one aspect is described an electrodeposition solution to electrodeposit a Group IB-Group VIA thin film that includes a a solvent; a Group IB material source; a Group VIA material source; and at least one complexing that forms a complex ion of the Group IB material. Also described are methods of electroplating using electrodeposition solutions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation in Part of U.S. patent applicationSer. No. 12/121,687 FILED May 15, 2008 entitled “SELENIUM ELECTROPLATINGCHEMISTRIES AND METHODS” (SP-049), and this application is aContinuation in Part of U.S. patent application Ser. No. 12/371,546filed Feb. 13, 2009 entitled “ELECTROPLATING METHODS AND CHEMISTRIES FORDEPOSITION OF COPPER-INDIUM-GALLIUM CONTAINING THIN FILMS” (SP-051),which claims priority to U.S. Provisional Application No. 61/150,721,filed Feb. 6, 2009, entitled “ELECTROPLATING METHODS AND CHEMISTRIES FORDEPOSITION OF COPPER-INDIUM-GALLIUM CONTAINING THIN FILMS” (SP-051P),and this application is a Continuation in Part of U.S. patentapplication Ser. No. 12/______ filed on Dec. 18, 2009 entitled “ENHANCEDPLATING CHEMISTRIES AND METHODS FOR PREPARATION OF GROUP IBIIIAVIA THINFILM SOLAR ABSORBERS” (SP-098) and this application is a Continuation inPart of U.S. patent application Ser. No. 12/______ filed on Dec. 18,2009 entitled “ELECTROPLATING METHODS AND CHEMISTRIES FOR DEPOSITION OFCOPPER-INDIUM-GALLIUM CONTAINING THIN FILMS”, (SP-101), all of which areexpressly incorporated herein by reference.

BACKGROUND

1. Field of the Inventions

The present inventions relate to selenium containing electrodepositionsolutions used to manufacture solar cell absorber layers.

2. Description of the Related Art

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon, whichis in the form of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce thecost of solar cells for terrestrial use. One way of reducing the cost ofsolar cells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se,Te, Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1-x)Ga_(x) (S_(y)Se_(1-y))_(k),where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employedin solar cell structures that yielded conversion efficienciesapproaching 20%. Absorbers containing Group IIIA element Al and/or GroupVIA element Te also showed promise. Therefore, in summary, compoundscontaining: i) Cu from Group IB, ii) at least one of In, Ga, and Al fromGroup IIIA, and iii) at least one of S, Se, and Te from Group VIA, areof great interest for solar cell applications. It should be noted thatalthough the chemical formula for CIGS(S) is often written asCu(In,Ga)(S,Se)₂, a more accurate formula for the compound isCu(In,Ga)(S,Se)_(k), where k is typically close to 2 but may not beexactly 2. For simplicity we will continue to use the value of k as 2.It should be further noted that the notation “Cu(X,Y)” in the chemicalformula means all chemical compositions of X and Y from (X=0% andY=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means allcompositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)₂ means thewhole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to1, and Se/(Se+S) molar ratio varying from 0 to 1.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1. A photovoltaic cell 10 is fabricated on a substrate 11, such asa sheet of glass, a sheet of metal, an insulating foil or web, or aconductive foil or web. An absorber film 12, which comprises a materialin the family of Cu(In,Ga,Al)(S,Se,Te)₂ is grown over a conductive layer13 or contact layer, which is previously deposited on the substrate 11and which acts as the electrical contact to the device. The substrate 11and the conductive layer 13 form a base 20 on which the absorber film 12is formed. Various conductive layers comprising Mo, Ta, W, Ti, and theirnitrides have been used in the solar cell structure of FIG. 1. If thesubstrate itself is a properly selected conductive material, it ispossible not to use the conductive layer 13, since the substrate 11 maythen be used as the ohmic contact to the device. After the absorber film12 is grown, a transparent layer 14 such as a CdS, ZnO, CdS/ZnO orCdS/ZnO/ITO stack is formed on the absorber film 12. Radiation 15 entersthe device through the transparent layer 14. Metallic grids (not shown)may also be deposited over the transparent layer 14 to reduce theeffective series resistance of the device. The preferred electrical typeof the absorber film 12 is p-type, and the preferred electrical type ofthe transparent layer 14 is n-type. However, an n-type absorber and ap-type window layer can also be utilized. The preferred device structureof FIG. 1 is called a “substrate-type” structure. A “superstrate-type”structure can also be constructed by depositing a transparent conductivelayer on a transparent superstrate such as glass or transparentpolymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)₂ absorberfilm, and finally forming an ohmic contact to the device by a conductivelayer. In this superstrate structure light enters the device from thetransparent superstrate side.

The first technique that yielded high-quality Cu(In,Ga)Se₂ films forsolar cell fabrication was co-evaporation of Cu, In, Ga and Se onto aheated substrate in a vacuum chamber. However, low materialsutilization, high cost of equipment, difficulties faced in large areadeposition and relatively low throughput are some of the challengesfaced in commercialization of the co-evaporation approach. Anothertechnique for growing Cu(In,Ga)(S,Se)₂ type compound thin films forsolar cell applications is a two-stage process where metallic componentsof the Cu(In,Ga)(S,Se)₂ material are first deposited onto a substrate,and then reacted with S and/or Se in a high temperature annealingprocess. For example, for CuInSe₂ growth, thin layers of Cu and In arefirst deposited on a substrate and then this stacked precursor layer isreacted with Se at elevated temperature. If the reaction atmosphere alsocontains sulfur, then a CuIn(S,Se)₂ layer can be grown. Addition of Gain the precursor layer, i.e. use of a stack such as a Cu/In/Ga stackedfilm precursor, allows the growth of a Cu(In,Ga)(S,Se)₂ absorber.

Sputtering and evaporation techniques have been used in prior artapproaches to deposit the layers containing the Group IB and Group IIIAcomponents of the precursor stacks. In the case of CuInSe₂ growth, forexample, Cu and In layers are sequentially sputter-deposited on asubstrate and then the stacked film is heated in the presence of gascontaining Se at elevated temperature for times typically longer thanabout 30 minutes, as described in U.S. Pat. No. 4,798,660. More recentlyU.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositinga stacked precursor film comprising a Cu—Ga alloy layer and an In layerto form a Cu—Ga/In stack on a metallic back electrode layer and thenreacting this precursor stack film with one of Se and S to form theabsorber layer. U.S. Pat. No. 6,092,669 described sputtering-basedequipment for producing such absorber layers.

Two-stage processing approach may also employ stacked layers comprisingGroup VIA materials. For example, a Cu(In,Ga)Se₂ or CIGS film may beobtained by depositing In—Ga-selenide and Cu-selenide layers in astacked manner and reacting them in presence of Se. Similarly, stackscomprising Group VIA materials and metallic components may also be used.In—Ga-selenide/Cu stack, for example, may be reacted in presence of Seto form CIGS. Stacks comprising metallic elements as well as Group VIAmaterials in discrete layers may also be used. Selenium may be depositedon a metallic precursor film comprising Cu, In and/or Ga through variousapproaches to form stacks such as Cu/In/Ga/Se and Cu—Ga/In/Se. Oneapproach for Se layer formation is evaporation as described by J. Palmet al. (“CIS module pilot processing applying concurrent rapidselenization and sulfurization of large area thin film precursors”, ThinSolid Films, vol. 431-432, p. 514, 2003) in their work that involvedpreparation of a Cu—Ga/In metallic precursor film by sputtering andevaporation of Se over the In surface to form a Cu—Ga/In/Se stack. Afterrapid thermal annealing and reaction with S, these researchers reportedformation of Cu(In,Ga)(Se,S)₂ or CIGS(S) absorber layer.

Evaporation is a relatively high cost technique to employ in large scalemanufacturing of absorbers intended for low cost solar cell fabrication.Potentially lower cost techniques such as electroplating have beenreported for deposition of Se films. Electroplating can be used fordepositing substantially pure Se thin films as well as for co-depositingSe with Cu, In and Cu metallic components. One specific method for theformer case involves depositing a metallic precursor comprising Cu andIn on a substrate and then electroplating a Se layer over the Cu and Incontaining layer to form a Cu—In/Se stack. This stack may then be heatedup to form a CuInSe₂ compound absorber.

Binary metal selenide film preparation by electrodepositon has beenreported in various publications. For example, Massaccessi et al.carried out In—Se electroplating from indium sulfate and selenious acidsolutions (“Electrodeposition of indium selenide In₂Se₃, J.Electroanalytical Chemistry, vol. 412, p. 95, 1996). Kemell et al.(“Electrochemical quartz crystal microbalance study of theelectrodepositon mechanism of Cu_(2-x)Se thin film”, ElectrochemicalActa, vol. 45, p. 3737, 2000) used a thiocyanate solution to depositcopper selenide thin films. In addition to preparation of substantiallypure selenium films and binary metal selenides, one-step electroplatingprocess has also been used to deposit the entire precursor film in theform of Cu—In—Se or Cu—In—Ga—Se. For example, CIGS films have beenprepared with electrochemical co-deposition method from acidic solutionscontaining CuCl₂, InCl₃, GaCl₃ and SeO₂ (U.S. Pat. No. 6,872,295). Theelectrochemical co-deposition of CIS films is also performed using asolution containing Cu²⁺, In³⁺, Se⁴⁺ and citrate salts, as reported mthe literature (Oliveira et al., “A voltammetric study of theelectrodeposition of CuInSe₂ in a citrate electrolyte”, Thin SolidFilms, vol. 405, p. 129, 2002).

Electrochemical deposition techniques have been developed to depositpure Se films in both amorphous and metallic crystalline forms. Seleniumcan assume four allotropic modifications in its solid state; amorphous(also called vitreous),

-monoclinic,

-monoclinic and a hexagonal (so-called metallic) phase. The amorphousselenium is composed of irregular arrays of selenium chain molecules,while the monoclinic modifications consist of Se ring molecules.Vitreous and monoclinic modifications of Se are insulators and they aregenerally red in color. The hexagonal phase of selenium is gray inappearance and therefore called “gray” selenium. Hexagonal modificationis a semiconductor due to the ordered arrangement of selenium chainsfacilitating electronic conduction. A. Von Hippel et al. (U.S. Pat. No.2,649,409) disclosed that gray crystalline metallic Se may beelectroplated using an acidic electrolyte composed of saturated seleniumdioxide in 9 molar H₂SO₄ at a temperature of 100° C. Since plating ofmetallic Se requires use of high temperature solutions and highly acidicsolution formulations, they are not very suitable for large scaleproduction.

Typically Se deposits obtained from low temperature solutions are ofamorphous nature. A. Graham et al. (“Electrodeposition of amorphous Se”,J. Electrochemical Society, vol. 106, p. 651, 1959) have establishedthat amorphous Se layers with thicknesses up to 500 nm can be platedusing acidic (pH 0.7-0.9) or alkaline (pH 7.5-8.0) electrolytes in thetemperature range between 20 to 40° C. A common problem associated withelectrodeposition of amorphous Se films is that the current platingprocesses are known to produce colloidal Se which is mostly producednear the cathode surface. These colloidal Se particles aggregate and getlarger in size with time. The formation of Se particles is not onlyobserved in electrodeposition of pure Se layers but also occurselectroplating of metal selenides such as In—Se, Ga—Se, Cu—Se, Cu—In—Se,Cu—Ga—Se etc. The generation of colloidal particles might also bepresent in plating applications where other group VIB elements such astellurium and sulfur are electrodeposited either in the form of pureelemental layers or co-deposited with Se such as sulfur-selenium layers,tellurium-selenium layers and sulfur-tellurium-selenium layers, orco-deposited with metals such as In, Cu and Ga, or co-deposited with Seand metals such as In, Cu and Ga.

From the foregoing, there is a need in the solar cell manufacturingindustry, especially in thin film photovoltaics, for betterSe-containing electrodeposition solutions to deposit high quality Secontaining films.

SUMMARY OF THE INVENTION

The present inventions relate to selenium containing electrodepositionsolutions used to manufacture solar cell absorber layers, and methods ofusing the same.

In one aspect is described an electrodeposition solution toelectrodeposit a Group IB-Group VIA thin film, comprising: a solvent; aGroup IB material source that dissolves in the solvent and provides aGroup IB material; a Group VIA material source that dissolves in thesolvent and provides a Group VIA material; and at least one complexingthat forms a complex ion of the Group IB material wherein such complexion dissolves in the solvent; an adhesion promoting agent; a corrosioninhibitor; and wherein the pH of the electrodeposition solution is inthe range of 1-13.

In another aspect is described a method of electrodepositing an adherentfilm comprising copper selenide alloy material on a conductive layer,comprising: providing an electrodeposition solution comprising asolvent, a copper ion source, a selenium ion source, at least onecomplexing agent and at least one adhesion promoting agent, the adhesionpromoting agent suppressing the formation of colloidal particles on thesubstrate and in the plating solution, wherein the electrodepositionsolution has a pH value in the range of 1-13; contacting theelectrodeposition solution with the surface of the conductive layer andan anode; establishing a potential difference between the anode and theconductive layer; and electrodepositing the copper selenide film on thesurface of the conductive layer.

In a further aspect is described a method of electrodepositing aprecursor on a conductive layer, comprising: electrodepositing a copperselenide film on the conductive layer using an electrodepositionsolution, the electrodeposition solution comprising a copper ion source,a selenium ion source, at least one complexing agent, at least oneadhesion promoting agent; and depositing a thin film including a GroupIB material, at least one Group IIIA material onto the copper selenidefilm, wherein the step of depositing uses a physical vapor depositiontechnique.

BRIEF DESCRIPTION OF THE DRAWING

These and other aspects and features of the present invention willbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a schematic view of a prior art solar cell structure;

FIG. 2A is a schematic view of a precursor stack electrodeposited on abase; and

FIG. 2B is a schematic view of a CIGS absorber layer formed when theprecursor stack shown in FIG. 2A is reacted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment, as describe herein, provide electrodeposition(also called electroplating or plating) solutions (also called baths orelectrolytes) and methods to co-electrodeposit (also calledelectrodeposit or electroplate or plate from now on) uniform, smooth,continuous and compositionally repeatable Group IB-Group VIA alloy ormixture films. The electrodeposition solution compositions and methodsof the present invention suppresses or eliminate the formation ofselenium particles in the electrodeposition solution, thereby allowingelectrodeposition of defect-free and highly adherent, low stress GroupIB-Group VIA alloy or mixture thin films at a high deposition rate inboth conformal and non-conformal fashions. Such films can be used in thepreparation of Group IBIIIAVIA compound semiconductors.

In one embodiment, for example, the Group IB material may be Cu and theGroup VIA material may be Se to electroplate a copper selenide, Cu—Sefilm. Such Cu—Se films may be used to form one or more layers of aprecursor stack of a Cu(In,Ga)(Se)₂ absorber layers. The stoichiometryor composition of such films, e.g. Group IB/Group VIA atomic ratio, maybe controlled by varying the composition of the electrodepositionsolution and by modification of the appropriate plating conditions. Forexample, the composition of Se in the deposited copper-selenide film maybe tailored to a specific composition or varied in a desirable mannerwithin the thickness of the film. The disclosed electrodeposition Cu—Seelectrodeposition solutions may be used to deposit a copper selenidefilms containing more than, for example, 95% Se at pH values lower than3 as well as deposit highly copper rich selenide at pH values higherthan 7.5. Through the use of embodiments described herein it is possibleto form micron or sub-micron thick Cu—Se films on conductive surfacesfor the formation of solar cell absorbers.

The electrodeposition solution of the present invention comprises asource of Group IB material, such as a copper source, a source of GroupVIA material such as a selenium source, at least one complexing agent,an adhesion promoting agent, a corrosion inhibitor such as multihydricalcohol, for example glycerol, a pH adjuster and a solvent such aswater. The pH range of the plating solution is between 1 and 13. The pHof the plating bath can be modified, for example, depending on thenature of the substrate, and the copper content of the copper selenidefilm.

A Cu—Se electrodeposition solution, or electrodeposition solution orsolution hereinbelow, of the present invention includes known solublecopper salts, for example, sulfates, chlorides, nitrates, oxides,hydroxides, tetrafluoroborates, citrates, acetate gluconate, phosphates,sulfamates, carbonates, copper amine salts or the like as the Cu source.A copper sulfate source may preferably be used as the Cu source in thisinvention. Cu source may comprise stock solutions prepared by dissolvingcopper metal into its ionic form in a suitable solvent such as water.The copper ions concentration may be in the range of 1-500 mM/L andpreferably 5-100 mM/L.

In the Cu—Se electrodeposition solution, selenium oxide may provide theGroup VIA material source although other Group VIA compounds orelemental Group VIA materials may also be used for this function as longas they dissolve in the electrodeposition solution. In certainembodiments, the Group VIA material source may be compounds of Se, S,and Te such as acids of Se, S, and Te as well as oxides, chlorides,sulfates, nitrates, perchlorides, and phosphates of Se, S, and Te. TheSe source may preferably include soluble selenium salts or acid orcombination of both, for example, sodium selenate and selenious acid.The selenium content in the Cu—Se electrodeposition solution may be inthe range of 50-1.5 M/L, preferably 75-700 mM/L.

The Cu—Se electrodeposition solution may include at least one complexingagent. The complexing agents may be water soluble complexing agents withcarboxylate and/or an amine group. A blend of two or more complexingagents can be included to provide full complexation of Group IB ionsincluding Cu ions. Complexing agents can be selected from organic acidssuch as citric acid, tartaric acid, maleic acid, malic acid, oxalic acidsuccinic acid, ethylenediamine (EN), ethylenediaminetetra acetic acid(EDTA), nitrilotriacetic acid (NTA), andhydroxyethylethylenediaminetriacetic acid (HEDTA). The salts of theseacids with alkali metals and alkali earth metals can be also used as thesource for complexing agents. Sodium citrate, potassium tartrate andcalcium oxalate can be used to provide citrate, tartrate and oxalatecomplexing ions, respectively. Ammonium salts of these complexing agentscan be also used to provide the complexing ions. For example, ammoniumcitrate, either in the form of ammonium citrate dibasic or in the formof ammonium citrate tribasic form was found to be a very useful sourcematerial to provide citrate complexing ions in the electrodepositionsolution. The complexing agent content of the electrodeposition solutionmay preferably be in the range of 5 mM/L to 0.55 M/L, more preferablybetween 0.1M to 0.4 M/L. When ammonium citrate is used as the complexingagent source, the ammonium citrate content of the electrodepositionsolution may vary between 0.05 M/L and 0.4 M/L, and preferably between0.08 M/L and 0.35 M/L. It is also possible to prepare thiselectrodeposition solution for other Group IB materials. Silver (Ag) orgold (Au) ion sources may also be added to Cu—Se electrodepositionsolutions to deposit layers including them. For example, in oneembodiment, an electrodeposition solution including Cu, Ag and Se may beprepared to electrodeposit Cu—Ag—Se layers. In this case, an additionalcompleting agent for Ag may or may not be added to the electrodepositionsolution.

The electrodeposition solution of the present invention might furtherinclude other compounds, specifically added to increase the conductivityof the electrodeposition solution. The conductivity improving agentsmight be selected from ammonium salts of inorganic acids, alkali metalsalts of inorganic acids, and alkali metal earth salts of inorganicacids. The conductivity improving agents might include but not limitedto ammonium sulfate, potassium sulfate, sodium sulfate, magnesiumsulfate, ammonium nitrate, potassium nitrate, sodium nitrate, magnesiumnitrate, ammonium chloride, potassium chloride, sodium chloride,magnesium chloride. For example, ammonium sulfate was found to be a veryuseful source material to high conductivity in the bath. Theconductivity improving agent content of the electrodeposition solutionmay preferably be in the range of 5 mM/L to 0.55 M/L, more preferablybetween 0.1M to 0.4 M/L. When ammonium sulfate is used as theconductivity improving agent source, the ammonium sulfate content of theelectrodeposition solution may vary between 0.05 M/L and 0.4 M/L, andpreferably between 0.08 M/L and 0.35 M/L.

The adhesion promotion agent added to the Cu—Se electro depositionsolution enhances Cu—Se film formation and its adhesion to the surfaceof the underlying layer during the electrodeposition process. Adhesionpromoting agents may be selected from a group of organic compounds withamine functional groups, which can form very strong chealation withcopper ions. Preferably, organic compounds with amine functional groups,which also contain at least one aminopropyl group are desirable becauseof their ability to form very stable complexes with copper ions. Anexemplary adhesion promoting agent preferably includes an organic aminemolecule with at least one nitrogen atoms, and preferably with opposingmultiple nitrogen atoms or nitrogen bearing group. For example, organiccompounds with piperazine functional groups may be used as adhesionpromoting agents in this invention. The adhesion promoting agents notonly improve the adhesion of the film but also largely suppress theformation of powdery or colloidal Se particles or aggregates that mightform on the cathode and within the electrolyte solution during theplating process. Therefore, they also provide a large increase in thecathodic efficiency for plated films. Examples of suitable adhesionpromotion agents of this invention included but not limited to1,4-Bis(3-aminopropyl) piperazine and N, N′ Bis(3-aminopropyl)ethylenediamine. Alternatively, any possible mixture of these adhesionpromoting agents may be used in the electrodeposition solution of thepresent invention. The amount of the adhesion promoting agents in theelectrodeposition solution is in the range of 0.01 mM/L to 200 mM/L,preferably in the range of 1.0 mM/L to 40 mM/L.

Another key constituent of the Cu—Se electrodeposition solution is anorganic compound, which acts as a corrosion inhibitor whenelectrodeposition is conducted on a galvanically more active surface,for example Cu—Se plating over In and Ga metal or alloy thin filmscomprising In and Ga metals or alloys comprising In and Ga. Such organiccorrosion inhibitors of the present invention might be selected frommultihydric alcohols. Specifically, glycerol was found to beparticularly useful for the Cu—Se electrodeposition solution. Theaddition of glycerol into the Cu—Se electrodeposition solution,suppresses/eliminates the dissolution or partial dissolution of somesubstrate thin film material during the electrodeposition step. Theglycerol enhances substrate wetting and improves coating uniformity bydispersing any particulates in the solution as well as suppressing theiradhesion to the substrate surface. When added at high concentrations, inthe presence of adhesion promoting compounds with piperazine functionalgroups, glycerol may also refine the microstructure of the plated Cu—Sefilm, by forming smaller grains in a uniform fashion with a good surfacecoverage. The concentration of the multihydric alcohols, such asglycerol in the present invention may vary between 20 mM/L to 500 mM/Land preferably between 30 mM/L to 250 mM.

The Cu—Se electroplating solution of the present invention might furtherinclude grain-refiners. To obtain a layer with a microstructure withfiner grains, inorganic additives with chloride, pyrophosphate andsulfite ions might be added to the electrodeposition solution. Thechloride additives may include known water soluble chloride compounds,for example, sodium chloride, ammonium chloride. The pyrophosphateadditives may include for example potassium pyrophosphate. The sulfiteadditives may include sodium sulfite. It is also preferable that theconcentration of the sulfite, or chloride, or pyrophosphate and theirvarious combinations does not exceed 1% of the Cu—Se electrodepositionsolution. Excessive amount of chlorides, pyrophosphates and sulfitesmight produce defective films and may lead to premature excessiveselenium precipitation in the electrodeposition cell.

The Cu—Se electrodeposition solutions of present invention may beprepared in a wide range of pH value between 1 and 13. The specific pHof the electrodeposition solution depends on factors such as the natureof the substrate, level of conformity desired in the film, and thecopper content of the copper selenide film. A pH range between 1 and 5,more preferably between 1.5 to 3, is suitable for plating operations todeposit Se rich Cu—Se films with a high Se/Cu ratio, greater than 10. Inthis acidic pH regime, the pH of the plating bath may be adjusted with apH adjuster, which could be as simple as an inorganic acid such assulfuric acid or sulfonic acid or a pH buffer couple chosen for the pHof the particular application. Increasing pH of the electrodepositionsolution allows deposition of Cu—Se films with higher Cu content. Ahighly Cu rich Cu—Se film may be deposited at pH values greater than7.5. The pH of the solution in the alkaline regime can be adjusted byaddition of sodium hydroxide, potassium hydroxide or ammonium hydroxide.Alkaline buffer couples could be also employed to adjust the pH. Suchalkaline pH buffer systems include but not limited to monopotassiumphosphate/dipotassium phosphate, boric acid/sodium hydroxide, sodiumbicarbonate/sodium carbonate, monosodium tellurate/disodium tellurate,monosodium ascorbate/disodium ascorbate, and dipotassiumphosphate/tripotassium phosphate. The pH value of the electrodepositonsolution also regulates the conformation of the Cu—Se film

Electrodeposited Group IBVIA layers using the electrodeposition solutioncan be employed in the preparation of precursor layers in a two-stageprocess where such precursor layers are reacted in a high temperatureannealing process. During reaction, sulfur and/or more selenium mightpresent to produce the desired compound form. FIG. 2A shows an exemplaryprecursor stack 100 which will be utilized below to describe varioususes of the electrodeposition solution of the present invention. In thisembodiment, the precursor stack 100 may include a multilayer structureincluding a first layer 102, a second layer 104 and an optional thirdlayer 106. In this embodiment, at least a portion of the precursor stack100 is electrodeposited using the Cu—Se electrodeposition solution ofthe present invention. During the process, initially, the first layer102 may be formed over a base 101 which may include a substrate 101A anda contact layer 101B formed over the substrate. The second layer 104 iselectrodeposited on the first layer 102 and the third layer 106 may beelectrodeposited on the second layer. Principles of theelectrodeposition process are well known and will not be repeated herefor the sake of clarity. The contact layer 101B may be made of amolybdenum (Mo) layer deposited over the substrate 101A or a multiplelayers of metals stacked on a Mo layer; for example, molybdenum andruthenium multilayer (Mo/Ru), or molybdenum, ruthenium and coppermultilayer (Mo/Ru/Cu). To form a contact layer having multi layers, forexample, Ru layer may be electrodeposited on the Mo layer, and similarlythe Cu layer may be electrodeposited on the Ru layer to form the contactlayer. The substrate 101A may be a flexible substrate, for example astainless steel foil, or an aluminum foil, or a polymer. The substratemay also be a rigid and transparent substrate such as glass.

As shown in FIG. 2B, in the following step, the precursor stack 100 isreacted in a reactor to transform it to an absorber layer 108 i.e. acompound layer. As will be described below, the composition of theprecursor stack determines the composition of the resulting absorberlayer or compound layer. In one embodiment, the Cu—Se electrodepositionsolution of the present invention may be used to prepare Group IBIIIAVIAcompound semiconductors such as CuInSe₂, CuGaSe₂ CuAlSe₂, Cu(In, Ga)Se₂, Cu(In, Al, Ga) Se₂, Cu(In, Al, Ga) (Se, S)₂ and Cu(In, Ga) (Se,S)₂, which can be used as solar absorber layers. In this case, the firstlayer 102 and the second layer 104 of the precursor stack may compriseGroup IB, Group IIIA and Group VIA materials, i.e., Cu, In, Ga and Se toform a Group IBIIIAVIA compound layer.

In this embodiment, the first layer 104 may be configured as a stackincluding a Cu-film, an In-film and a Ga-film, which will be shown withCu/In/Ga insignia hereinbelow. This and similar insignia will be usedthroughout the application to depict various stack configurations, wherethe first material (element or alloy) symbol is the first film, thesecond material symbol is the second film deposited on the first filmand so on. For example, in the Cu/In/Ga stack: the Cu-film, as being thefirst film of the stack, may be electrodeposited over the contact layeror another stack; the In-film (the second film) is electrodeposited ontothe Cu-film; and the Ga-film (the third film) is deposited onto theIn-film. In the first layer 102, the order of such films 102 may bechanged, and the first layer 102 may be formed as a Ga/Cu/In stack orIn/Cu/Ga stack. Furthermore, the first layer 102 may be formed as astack of four films, such as Cu/Ga/Cu/In or Cu/In/Cu/Ga. In anotherembodiment, the first layer 102 may be formed as a (Cu—In—Ga) ternaryalloy film or as a stack including (Cu—In) binary alloy film and (Cu—Ga)binary alloy film. Such alloy binary or ternary alloy films may have anydesired compositions. The first layer 102 may be formed by any possiblecombinations of the above given stacks of films, binary films andternary alloy films.

Referring back to FIG. 2A, the second layer 104 includes a Cu—Se alloyfilm electrodeposited using the electrodeposition solution of thepresent invention. Alternatively, the second layer may include a mixturefilms including at least one of the films included in the first layerand at least one film of Cu—Se. The third layer 106 is an optional layerto add more Se to the precursor stack 100. If more Se is desired in theprecursor, it could be added as the third layer 106. Preferably, thesecond layer includes a Cu—Se film.

Alternatively, the Cu—Se electrodeposition solution of the presentinvention may be used to form precursors for absorber layers comprisingas ternary compounds such as CuGaSe₂, CuInSe₂, CuAlSe₂. In thisembodiment, the first layer 102 may comprise a Group IIIA material, suchas one of In, Ga, or Al, and the second layer 104 comprises (Cu—Se)film. Precursor stacks including Ga/(Cu—Se), In/(Cu—Se) and Al/(Cu—Se)can be prepared by electrodepositing the (Cu—Se) layer over Ga, In andAl films, respectively, which are deposited on, for example, a contactlayer such as Mo. Such layers can be later reacted at high temperaturein a S and/or additional Se-containing environment to form CuGaSe₂,CuInSe₂, CuAlSe₂, respectively.

The copper selenide (Cu—Se) film of the present invention may bedeposited at very high deposition rates. The electrodeposition currentdensity may be greater than 30 mA/cm² as compared to typical 2 mA/cm² ofthe prior art. Cu—Se film is very uniform and exhibits excellentadhesion and around pH=2. The Cu—Se film composition is independent ofplating current density between 5 and 30 mA/ cm². The copper content ofthe Cu—Se film may be varied between less than 6% and more than 50% by,for example: adjusting pH alone; adjusting copper content of the Cu—Seelectrodeposition solution; adjusting selenium content of bath; and theaddition of sulfonic acid or EDTA. The Cu—Se electrodeposition solutionis capable of electrodepositing a film including very high Se content.For example, the Cu—Se electrodeposition solution can deposit copperselenide films with less than 6% copper on indium substrates withminimal indium loss. For copper rich continuous and adherent Cu—Sefilms, the selenous acid may be replace with sodium selenate and Cu—Sefilms plated at pH between 7 and 13, and preferably at a pH between 8and 11.

EXAMPLE

An exemplary Cu—Se electrodeposition solution was prepared andelectrodeposited on a thin film of In with approximately 3000 Angstromthickness, which was electroplated over a Mo/Cu containing stack onstainless steel substrate. The Cu—Se electrodeposition solution includesthe following composition: 5 g/L copper sulfate pentahydrate; 30 g/Lsodium citrate; 60 g/L sodium sulfate; and 50 g/L selenious acid. The pHof the electrodeposition solution was adjusted to 2 by adding sulfuricacid. During the electrodeposition, a potential difference isestablished between the metallic surface and an anode such as Pt coatedTi electrode. A plating current density of about 30 mA/cm² is appliedfor 60 seconds. The electrodeposition process resulted in a Cu—Se filmhaving a thickness of about 330 nm. Generation of Se particles oraggregates was observed emanating from the cathode surface during thedeposition step. In the following experiment, the addition of 1.25ml/liter 1,4-Bis(3-aminopropyl) piperazine as the adhesion promotingagent to this electrodeposition solution produced an extremely adherentfilm and no visible Se particle or aggregate generation during theplating process. This provided a large increase in the cathodicefficiency yielding a thickness of 562 nm.

The Cu—Se electrodeposition solutions may also be used to form gradedCu—Se films, or to vary the copper composition profile, within the filmthickness, by varying electrodeposition current density during theelectrodeposition. The plating current density may be graded from a highcurrent density and terminated with a much lower current density or viceversa. The much lower current density Cu—Se film containingcomparatively higher copper percentage than the film plated at highercurrent density. The current may be pulsed between a higher and a lowcurrent density for desirable time intervals to coat a laminatedselenide films containing with different copper concentrations. Forexample, at a fixed pH, a Cu—Se layer may be deposited at 25 mA/sq.cmfor 60 s, the current density is the reduced to 2.5 mA/sq.cm for 30 s.The selenide film deposited at 25 mA/sq.cm may contain about 8% copperand the film deposited at 2.0 mA/sq.cm may contain about 14% copper inthe layer, producing a Cu—Se alloy with a copper rich surface. Thelaminate may be symmetrical or assymetrical, the initial alloy coatingmay be relatively copper rich or copper poor with respect to thesubsequent Cu—Se material coated.

The Cu—Se electrodeposition solution may be adapted for multi-celloperations, where individual cell may dispose variant chemistries of theCu—Se electrodeposition solution. Such multi-cell plating approach maybe used to vary the copper composition profile within the coatingthickness. The substrate may be plated sequentially or non-sequentialyacross the various plating cells to achieved the desired alloy profile.The deposited Cu—Se films exhibit low stress characteristics. Forexample, more than 3 micron Cu—Se film may be electrodeposited on anindium film without film cracking or any local delamination. The Cu—Salloy films of this invention may be plated galvanostatically, the saidalloys may also be deposited potentiostatically or various combinationsof current and voltage mode.

The Cu—Se film of the present invention may be used to replace asignificant portion of the evaporated selenium in the precursor,resulting in a very significant reduction in the operating cost ofselenium evaporators. In another embodiment of this invention, theelectrodeposited Cu—Se film may be covered or coated with other thinfilm materials, for example, sodium fluoride, indium, gallium, binary orternary photovoltaic alloy material using PVD deposition techniquesprior to the selenization step. As mentioned before, the Group IBmaterial may also include silver (Ag) or gold (Au). For example

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. An electrodeposition solution to electrodeposit a Group IB-Group VIAthin film, comprising: a solvent; a Group IB material source thatdissolves in the solvent and provides a Group IB material; a Group VIAmaterial source that dissolves in the solvent and provides a Group VIAmaterial; and at least one complexing that forms a complex ion of theGroup IB material wherein such complex ion dissolves in the solvent; anadhesion promoting agent; a corrosion inhibitor; and wherein the pH ofthe electrodeposition solution is in the range of 1-13.
 2. Theelectrodeposition solution of claim 1, wherein the Group IB materialsource comprises a copper source and the Group VIA material comprises aselenium source.
 3. The electrodeposition solution of claim 2, whereinthe copper source of copper ions comprises at least one of dissolvedcopper metals and dissolved copper salts, wherein the copper saltsinclude copper-chloride, copper-sulfate, copper-acetate, copper-oxide,copper-hydroxide, copper-nitrate, copper-phosphate,copper-tetraflouroborate, copper-citrate, copper-gluconate,copper-sulfamate, and copper-carbonate.
 4. The solution of claim 1,wherein the Group Se material source comprises at least one of dissolvedelemental Se, acids of Se, and dissolved Se compounds, wherein the Secompounds include oxides, chlorides, sulfates, nitrates, perchloridesand phosphates of Se.
 5. The solution of claim 4 wherein the complexingagent includes at least one of a carboxylate functional group and anamine functional group.
 6. The solution of claim 5 wherein thecomplexing agent comprises one of an acid and an alkali metal salt ofthe acid and an ammonium salt of the acid, and wherein the acidcomprises one of tartaric acid, citric acid, acetic acid, malonic acid,malic acid, succinic acid, ethylenediamine (EN), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), andhydroxyethylethylenediaminetriacetic acid (HEDTA).
 7. The solution ofclaim 6 wherein the complexing agent comprises an alkali metal salt ofthe acid is selected from the group of sodium and potassium saltstartaric acid, citric acid, acetic acid, malonic acid, malic acid,succinic acid, ethylenediamine (EN), ethylenediamine tetraacetic acid(EDTA), nitrilotriacetic acid (NTA), andhydroxyethylethylenediaminetriacetic acid (HEDTA).
 8. The solution ofclaim 6 wherein the complexing agent comprises an ammonium salt of theacid is selected from the group of ammonium salts of tartaric acid,citric acid, acetic acid, malonic acid, malic acid, succinic acid,ethylenediamine (EN), ethylenediamine tetraacetic acid (EDTA),nitrilotriacetic acid (NTA), and hydroxyethylethylenediaminetriaceticacid (HEDTA).
 9. The electrodeposition solution of claim 4, wherein theamount of the complexing agent is in the range of 5 mM/L to 0.55 M/L.10. The electrodeposition solution of claim 1, wherein the adhesionpromoting agent comprises organic compounds with at least one aminefunctional group and at least one aminopropyl group.
 11. Theelectrodeposition solution of claim 11, wherein the adhesion promotingagent comprises at least one of 1,4-Bis(3-aminopropyl) piperazine andN,N′ Bis(3-aminopropyl) ethylenediamine and any possible mixture ofthese.
 12. The electrodeposition solution of claim 7, wherein the amountof the adhesion promoting agent is in the range of 1.0 mM/L to 40 mM/L.13. The electrodeposition solution of claim 1, wherein the corrosioninhibitor comprises a multihydric alcohol.
 14. The electrodepositionsolution of claim 13, wherein the corrosion inhibitor is glycerol. 15.The electrodeposition solution of claim 13, wherein the amount of thecorrosion inhibitor is in the range of 20 mM/L to 500 mM/L.
 16. Theelectrodeposition solution of claim 1 further comprising a grain refinerincluding one of chloride, pyrophosphate and sulfite.
 17. Theelectrodeposition solution of claim 1 further comprising a conductivityimproving agent including ammonium sulfate.
 18. The electrodepositionsolution of claim 1, wherein the amount of the conductivity improvingagent is in the range of 5 mM/L to 0.55 M/L.
 19. A method ofelectrodepositing an adherent film comprising copper selenide alloymaterial on a conductive layer, comprising: providing anelectrodeposition solution comprising a solvent, a copper ion source, aselenium ion source, at least one complexing agent and at least oneadhesion promoting agent, the adhesion promoting agent suppressing theformation of colloidal particles on the substrate and in the platingsolution, wherein the electrodeposition solution has a pH value in therange of 1-13; contacting the electrodeposition solution with thesurface of the conductive layer and an anode; establishing a potentialdifference between the anode and the conductive layer; andelectrodepositing the copper selenide film on the surface of theconductive layer.
 20. The method of claim 19, wherein the conductivelayer is one of indium, gallium, selenium copper and their alloys. 21.The method of claim 19 wherein the copper ion source is copper sulfatepentahydrate, and the selenium ion source is selenium oxide.
 22. Themethod of claim 19, wherein said adhesion promoting agent comprisesorganic compounds with at least one amine functional group and at leastone aminopropyl group.
 23. The method of claim 19, wherein the step ofelectrodepositing includes varying electrodeposition current densityduring the electrodeposition.
 24. The method of claim 19, wherein thestep of electrodepositing includes pulsing the current density between ahigh and a low current density for desirable time intervals toelectrodeposit laminated selenide films containing different copperconcentrations.
 25. The method of claim 19 wherein said copper selenidefilm has a graded copper composition within the film thickness.
 26. Themethod of claim 19 wherein the step of electrodepositing is performedusing one of galvanostatic electrodeposition, potentiostaticelectrodeposition , and various combinations of current and voltagemode.
 27. A method of electrodepositing a precursor on a conductivelayer, comprising: electrodepositing a copper selenide film on theconductive layer using an electrodeposition solution, theelectrodeposition solution comprising a copper ion source, a seleniumion source, at least one complexing agent, at least one adhesionpromoting agent; depositing a thin film including a Group IB material,at least one Group IIIA material onto the copper selenide film, whereinthe step of depositing uses a physical vapor deposition technique. 28.The method of claim 27 wherein the thin film further includes a sodiumsalt.
 29. The method of claim 27 wherein the Group IB material includescopper, and the Group IIIA material includes gallium and indium.